Physiological sensors are used in medical applications to help doctors diagnose, monitor, and treat patients. The sensors use spectroscopy to provide valuable information about the body tissue. Spectroscopy generally refers to the dispersion of light as it travels through a medium. Light in some regions of the electromagnetic spectrum may disperse differently than light in other regions of the electromagnetic spectrum. For instance, light in the near-infrared region of the electromagnetic spectrum may disperse differently when traveling through body tissues than light in other regions.
A physiological sensor employing near-infrared spectroscopy may be used to detect characteristics of various body tissues by transmitting and receiving near-infrared light through the body tissue, and outputting a signal to a controller that provides valuable information about the body tissue. The information may be used by a doctor to diagnose, monitor, or treat the patient. In some instances, it is necessary to irradiate a single body tissue with different wavelengths of light, and to detect the light at multiple locations. Therefore, multiple sensors are placed in multiple locations, and the sensors are configured to sequentially operate, which requires a long sampling time. However, if too much light is absorbed or dispersed, or if the sampling time is not long enough, the near-infrared light will either not be received or the signal will be too weak to be reliable. Furthermore, a cable connecting a light detector to a controller may cause additional signal losses.
Previous attempts to correct for the weak signal include providing an amplifier with a feedback resistor that minimizes noise created by the amplifier. However, making the value of the feedback resistor too high could severely degrade the performance of the amplifier. In addition, the input impedance of the amplifier may have the characteristics of an inductor. Thus, the light detector and cable capacitance in combination with the amplifier result in a resonant circuit (such as an inductor-current tank) that oscillates. To dampen these oscillations, a capacitor is usually connected in parallel with the feedback resistor and, additionally, the capacitor reduces the bandwidth of the amplifier.
Accordingly, a physiological sensor is needed that provides more accurate and reliable data when too much light is absorbed or dispersed by the medium, or when the sampling time is too short to be reliable. In addition, a physiological sensor is needed that prevents significant signal losses caused by cables that connect the light detector to the controller.